Streamflow responses to potential changes in coniferous forest species composition and climate warming in the Western U. S

Background/Question/Methods

Recent studies suggest that forests in the Western U.S. are experiencing increases in background mortality rates. There is evidence of increases in drought-related dieback and disturbance losses due to wildfire and insects. These changes may accelerate climate-driven changes in forest structure and composition. Since streamflow patterns are correlated to vegetation water use, we hypothesize that replacement tree populations will impact the timing and amount of streamflow and that a warming climate will amplify those changes. In this model-based study, we explore how species change affects hydrologic responses in coniferous forests located in Oregon’s Western Cascades and northern Colorado’s Front Range. Specifically, we use the Regional Hydrologic Simulation System (RHESSys) to simulate forest growth, transpiration, evapotranspiration, and streamflow in the H.J. Andrews LTER, OR and in Big Thompson Watershed, CO. Scenarios examine varying key species in both watersheds, including Douglas fir (Pseudotsuga menziesii), ponderosa pine (Pinus pondersoa), and lodgepole pine (Pinus contorta). We examine how spatially explicit distributions of species affect the hydrologic response. The watershed scale responses of species variability are first examined using historical climate records. Then simple warming scenarios of 2- and 4oC are applied to examine the individual and synergistic responses of species change and climate variability. 

Results/Conclusions

Our results show that if replacement occurs only in the riparian zone, a broadleaf replacement of a coniferous species leads to increases in estimates of total watershed annual net primary productivity and transpiration at a maximum of 30% and 200%, respectively. Warming scenarios and species change scenarios result in streamflow reductions up to 25% in OR and 35% in CO during the first 10 years post-disturbance. The timing of streamflow changes differs for warming scenarios relative to species change scenarios. In warming scenarios, the largest reductions occur in early spring. Species change influences streamflow during the late summer. Both geographic regions are sensitive to temperature and species change but CO’s more arid and continental climate lends to greater sensitivity of modeled summer streamflow to these changes.